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BIG PHYSICS, BIG QUESTIONS –

The day the tower almost lost its power

By BARRY FOX

BRITISH TELECOM’S Tower, the London-based nerve centre of Britain’s
telecommunications network, teetered on the brink of electrical collapse
last month because of a failure in its supposedly secure power supply.

BT is cagey about details and the full story is only now leaking out.
If asked directly, the company will admit that on the morning of 10 November
500 private telephone circuits, carrying speech, electronic text and computer
data between businesses around Britain, were shut down for ‘several hours’.

But BT has not yet talked about how close the tower came to shutting
down all terrestrial and satellite TV broadcasting in Britain. This would
have blacked out all outside broadcasts, such as sports events, and cut
the satellite links on which TV stations round the world rely to exchange
news pictures.

The Telecom Tower is 189 metres tall, and was completed in 1965. A battery
of microwave dish aerials, located above 16 floors of electronic equipment,
communicates with relay station towers dotted around Britain at 50-kilometre
intervals. The tower also communicates by microwave with BT’s satellite
earth stations.

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In all, the tower can now handle nearly 150 TV channels or 200 000 telephone
conversations. A telephone exchange at the base of the tower handles a quarter
of all the telephone calls made in London. The tower also carries government,
military and defence communications links.

BT has always worried about security at the tower. A bomb exploded near
the top in 1971 and since then the building has been closed to the public.
As another security measure, the power supply for the electronics is not
taken direct from the mains. Instead, the tower’s electronic equipment takes
its power from banks of low-voltage batteries which are continually charged
from the mains. So if the mains supply fails, the circuits keep on working
until the batteries go flat.

But on the morning of Friday 10 November, one of three battery chargers,
each with a capacity of around 500 amps, failed. This overloaded a second
charger which started to smoke. All the TV stations in Britain were warned
at 10 am that the Tower had only seven hours’ power left. Throughout the
day, the company’s engineers desperately shed load from the tower, by disconnecting
as many circuits as possible. The TV stations set up alternative landline
links.

The engineers feverishly replaced the burnt-out transformer coils in
time to get the batteries charging again before the seven hours ran out.
The public never knew what nearly happened. BT will say only that ‘steps
have been taken’ to try to ensure that it does not happen again.